Propylene Glycol

What it is

Propylene glycol binds water, keeps formulations supple and dissolves substances that would not dissolve in pure water. It appears in eight products in this lexicon, from shampoos through styling products to hair colour and straightening product.

The name regularly prompts the objection that this is antifreeze. That is even true — propylene glycol is used in antifreeze products. But there it is precisely the harmless replacement for ethylene glycol, a different molecule, which caused the known poisoning cases. The point that is actually documented for propylene glycol is quite another: it is a comparatively common contact allergen.

Why

• Why the classification reads “low risk” and not “very low”: in an evaluation by the North American Contact Dermatitis Group over ten years, 23,359 people were tested. 810 of them, that is 3.5 per cent, showed an allergic reaction. In 88.3 per cent of these reactions a current link to the presenting complaint was seen, and the most common source was personal care products. Propylene glycol thus lies well above phenoxyethanol and ethylhexylglycerin.
• The antifreeze objection is literally correct and nevertheless says nothing. Where else a substance occurs tells us nothing about its tolerability on the skin. There is also a mix-up at work: the poisoning cases that gave antifreeze its reputation are due to ethylene glycol. Propylene glycol is the substance used there precisely because it is less hazardous.
• Irritation and allergy are hard to tell apart with this substance, and that is itself a finding. The majority of skin reactions to propylene glycol are irritant rather than allergic in nature, and the correct test concentration is disputed among specialists: it has to be high enough to reveal an allergy without irritating in itself. In 2018 the substance was named allergen of the year by the American contact dermatitis society.
• For the salon the distribution across the range is telling. Propylene glycol appears here not only in rinse-off shampoos but also in gel, definer, hair colour and straightening product — that is, in products that stay on the skin and in ones where the scalp is under strain anyway. Both raise the significance of the finding compared with a purely rinse-off use.
• The stated origin remains unchanged. The short field says “synthetic or plant-derived”, and that is correct: both routes are used industrially, and neither can be read from the ingredient list. Origin has no effect on the assessment in any case — the contact allergy is directed at the molecule, not at its history.

When the assessment differs

• Known contact allergy to propylene glycol: then the product is unsuitable. The substance appears under several names, which makes avoidance harder.
• Atopic dermatitis: in an evaluation in children, those affected showed positive test results for propylene glycol more often than children without atopic dermatitis.
• Corticosteroid creams and ointments: propylene glycol is a frequent excipient there. Anyone reacting to such a preparation should keep the excipient in mind — judging the individual case belongs in medical hands.
• Products that stay on skin and hair: with gel, definer and straightening product the contact time does not end with rinsing.
• Occupational contact: daily use on many heads means a higher total exposure on the hands.

Open questions

• Concentration in the specific product: not available. It matters particularly here, because the irritant effect is dose-dependent.
• Separating irritation from true allergy: not cleanly possible with the available figures. The test concentration is disputed among specialists, and some of the positive results are likely to be irritant in origin.
• Transferability of the North American figures to Europe: not checked. Test series at dermatology clinics also reflect a pre-selected group.
• Share of cosmetics in total exposure: open. Propylene glycol is also widespread in medicines, food and household products.
• Regulatory status: not checked; statements on use and maximum levels are not quoted second-hand.

Sources for this assessment

• Positive patch-test reactions to propylene glycol — a retrospective cross-sectional analysis from the North American Contact Dermatitis Group, 1996 to 2006. Auswertung von 23.359 getesteten Personen. Begutachtete Fachliteratur, abgerufen am 10.09.2026.
• Propylene Glycol — Allergic Contact Dermatitis and Topical Corticosteroids. Skin Therapy Letter. Übersichtsbeitrag zur Unterscheidung von Reizung und Allergie, zur umstrittenen Testkonzentration und zu den industriellen Verwendungen des Stoffes. Abgerufen am 10.09.2026.

Related substances

Applies to: Humectants

• Glycerin — Moisturising, humectant
• Panthenol — Moisturising, conditioning (provitamin B5)

Further substances from the same group for which a long-form assessment already exists.